The present invention relates to fixation devices used in orthopaedic surgery and particularly to devices used for the reduction of fractures or the positioning of bones by means of a plate attached to a bone or bone fragment in one region and secured to a rod which attaches to a cable, wire, plate or screw fastened in another region. The rod thus attaches between two bone regions for effecting stabilization, positioning, reduction or fixation of the bones.
A number of such mechanisms are known, among which should be mentioned the Harms T-plate which employs a split or slotted bolt, the head of which slides in a slot of a plate that is attached to a bone or bone fragment. The plate accepts the slotted bolt from the bottom and has several channels or grooves extending in different directions in the plate to allow positioning and alignment of the bolt along any one of the distinct channels. In use, a connecting rod fits through the slotted bolt and is captured by a nut which, when tightened, locks the bolt in its position in the channel, and secures the rod in the slot. In general, the system employs a slotted bolt with a square flange at its base so that each of the channels defines orientation of the rod-receiving slot of the outwardly protruding portion of the bolt. The plate thus provides a range of linear positions along several discrete lines, each at a fixed angular orientation, for the rod anchor point.
In addition to such plates, for posterior cervical fixation there also exists a number of eye screws that screw directly into the bone at a single fixed position. In these screws, the eye structure generally is an open slot or other rod-receiving open form adapted to receive the rod therein before being closed by a cap. The cap may be a conventional threaded locking nut, or in some constructions may be a dovetailed cap segment which slides in and wedges against the rod to secure the rod while closing the receiving slot at its open end. Such eye screws may also, in some constructions, be employed to secure a plate to the bone in addition to gripping the stabilization rod. When so used, the plate serves to strengthen the attachment and distribute the stresses coupled by the anchoring screw.
One variant of a rod-receiving fixation screw is the Moss-Miami polyaxial screw, as shown in U.S. Pat. No. 5,672,176. In that device, the screw has a spherical head. A slotted rod-holding cap structure having a conically tapering inner surface fits about the outside of the spherical head in the manner of a ball and socket joint. The rod-holding cap structure is internally threaded and is provided with a number of shaped packing or pressure-bearing inserts with an overall structure that tightens about the spherical screw head as the cap is drawn upward forcing the head down the cone angle. The cap may be rotated before tightening on the spherical head, so this clamping connection allows the rod-holding member to be bolted down and fixed with its slot oriented at an arbitrary angle in rotation about the axis of the screw. The rod fits through the slot in the holding cap structure and is secured by tightening a bolt into the threaded cap. The unit comes as a preassembled device with the packing or pressure-bearing members positioned internally about the ball end of the screw and held by swaging part way up the cap. Tightening of the cap against the rod then draws the conical outer holding body upward against the ball, fixing the slot orientation with the rod in position.
When the underlying bone has sufficient integrity, such individual eye screws offer great flexibility in rod orientation in one plane. Also, when a bone plate secured by multiple screws is necessary, the Harms plate offers a range of clamping point translational positions with a discrete set of angular orientations for connecting a cable, fixation rod or reduction rod. However, each of these systems has its own limitations as to convenience, or as to the range of position or orientation, or to the degree of loading that it may accommodate.
Accordingly, it would be desirable to provide a bone plate and rod junction system of adjustable angulation.
It would also be desirable to provide a multi-axis rod connection that is freely positionable along a slotted plate.
It would also be desirable to provide a multi-axis rod connection in which the components are preassembled to be installed as a unit during surgery.
One or more of the foregoing desirable ends are achieved in accordance with the present invention by a rod junction mechanism including a slotted bolt, a rod support platform that fits over the bolt, and a nut which tightens down to secure a rod in the slot on the support platform. The base of the bolt is configured to ride in a slot or a counter-bored aperture or channel in a plate or offset tab and is round so it rotates freely in the plate, while the support platform has the form of a generally annular and thick washer that lies over the base of the bolt to sandwich the plate therebetween and clamp firmly in position. The washer has an upper surface possessing a rounded groove on which the rod lies and a lower surface with a step collar that extends within and buttresses the walls of the opening in the plate. The plate is sandwiched between the base of the bolt and the support platform, distributing stress over a wide area while providing a strong anvil to support the rod.
In one embodiment the support platform has lateral openings that extend radially to a depth close to, but not through, its radially inner wall surface so as to leave a thinned wall portion. The bolt is placed through the plate and the platform is swaged to the bolt at the thinned wall portion so as to capture the bone plate therebetween, providing a single-piece assembly for convenience of handling and installation. In this configuration, the bolt may move freely within the bone plate opening as a captive assembly, easing placement during surgery by keeping all the components together. The swaging fixes alignment of the groove of the support plate form along the direction of the slot of the bolt, while leaving both pieces free to rotate, and when the plate opening is a slot, to translate along the slot of the bone plate. Rather than swaging, a preferred embodiment implements a one-piece assembly by providing a circumferential groove and a corresponding ridge on the bolt shaft and the support platform, to function as a snap ring and retain the two parts together during handling and use. In use, once the rod is positioned in the upwardly extending bolt slot, a locking nut or cap, which may be of conventional type, threads onto the bolt. The nut forces the rod against the support platform which, in turn, pushes downward against the bone plate while the nut also pulls the bolt upward, forcing the bolt base flange against the bottom of the plate. The bottom surface of the support platform, the plate-facing surface of the bolt flange, and/or the surface(s) of the plate may be roughened or toothed so as to enhance gripping and increase resistance to rotational or lateral movement of the bolt once the rod has been positioned and the nut torqued down.